The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Fuel cells create an electromotive force across an electrolyte by reacting a fuel, typically hydrogen, at an anode disposed on a first side of the electrolyte, and an oxidant, typically oxygen at a cathode disposed on a second side of the electrolyte.
In portable fuel cell systems, either hydrogen gas or hydrogen and other molecules reformed from hydrocarbons can be utilized for the anode reactions, and oxygen from the atmosphere can be utilized for the cathode reactions. The hydrogen and the hydrocarbons can be stored in a suitable holding tank and transported along with the fuel cell system. Hydrogen gas has a high energy-to-weight ratio but a low energy-to-volume ratio when packaged into a suitable holding tank as compared to hydrocarbon fuels. Therefore, a fuel cell utilizing hydrogen gas requires larger volumes and weights of stored fuel than a fuel cell utilizing hydrocarbon fuel to provide equivalent amounts of energy. Compressing hydrogen gas is inefficient due to the high levels of energy required to compress hydrogen. Further, compressed gas hydrogen storage requires storage containers having structural components with sufficient strength to retain the hydrogen under the desired pressure levels. The structural components add significant weight to compressed gas hydrogen storage containers, thereby offsetting efficiencies gained by the high energy per unit weight that is storable by hydrogen gas.
Alternative hydrogen storage methods include solid state storage and cryogenic liquid storage. Currently, solid-state hydrogen storage materials such as metal hydrides and fullerene-based materials have shown only low levels of hydrogen storage capacity, for example, reversible hydrogen storage levels of less than three weight percent and require costly materials. Cryogenic liquid storage of hydrogen requires costly equipment and utilizes high amounts of energy to maintain the sufficiently low temperatures required to provide hydrogen in a liquid state.
Another method of hydrogen storage includes chemical hydrides which are also limited in the ability to package hydrogen in a manner that provides equivalent amounts of energy as hydrocarbon fuel sources.
Due to limitations in current hydrogen storage methods, utilizing hydrocarbon-based fuel in fuel cells can provide advantages over utilizing hydrogen stored in molecular and solid-state form. Hydrocarbon-based fuel, as used herein, refers to any of a broad range of molecules containing hydrogen and carbon utilized in fuel and can include oxygenated hydrocarbons such as alcohols and glycols. Hydrocarbon fuels have high energy-to-volume ratios when compared to hydrogen gas and can be stored utilizing inexpensive storage containers when compared to compressed gas or liquid hydrogen. Further, hydrocarbons have a high energy-to-weight ratio and can be stored utilizing inexpensive storage systems when compared to solid-state hydrogen storage systems.
Reactants for the fuel cell including hydrogen and carbon monoxide can be liberated from hydrocarbon fuels in a fuel reformer. The fuel reformer can comprise catalyst material that catalyzes the reaction between oxygen and the hydrocarbon fuel to partially oxidize the hydrocarbon fuel and generate hydrogen. Atmospheric oxygen can be provided to the fuel reformer. Further, water vapor present in the fuel reformer can react with carbon monoxide fuel to generate hydrogen.
Certain standard hydrocarbon fuels contain substances that can poison or can reduce operational efficacy of fuel cell components. For example, diesel fuel and military JP-8 fuel can include sulfur containing molecules that can degrade fuel cell anode materials.
Further, reformed hydrocarbon fuel contains other substances that can reduce the operational efficacy of the fuel cell. In addition to hydrogen molecules that can be oxidized at the anode of the fuel cell to form water, the reformed product gas can include fully oxidized molecules such as carbon dioxide that are not reacted by the fuel cell anode. Further, atmospheric air utilized as the internal reformer oxygen source comprises about eighty percent nitrogen, which is not reacted by the fuel cell anode. The theoretical potential of the half reaction at the anode can be modeled by the Nernst Equation, depicted as Equation 1 below:
                    E        =                                            E              O                        ⁡                          (                              T                ,                P                            )                                -                                    RT              zF                        ⁢                          ln              (                                                P                                                            H                      2                                        ⁢                    0                                                                                        P                                          H                      2                                                        ·                                                            P                                              O                        2                                                                                                        )                                                          (        1        )            wherein,                E=Theoretical Potential        EO(T,P)=Ideal Potential as a function of temperature and pressure        R=Ideal gas law constant        T=Temperature        z=number of electrons transferred at the electrode        F=Faraday constant        PH2O=Partial pressure of water        PH2=Partial pressure of hydrogen        PO2=Partial pressure of oxygen        
As shown in Equation 1 above, nitrogen and fully oxidized molecules such as water and carbon dioxide within the reformed fuel reduce the partial pressure of hydrogen or other fuel cell reactant species at the fuel cell anode and therefore reduce the theoretical efficiency of the fuel cell.